Not applicable.
Not applicable.
BACKGROUND OF THE INVENTION
The present invention relates to dynamic braking of non-regenerative AC drives and more particularly to a method and an apparatus that facilitates drive braking to allow full use of a dynamic brake throughout essentially an entire braking cycle.
Power plants are linked to power consuming facilities (e.g., buildings, factories, etc.) via utility grids designed so as to be extremely efficient in delivering massive amounts of power. To facilitate efficient distribution, power is delivered over long distances as low frequency three-phase AC current.
Despite being distributable efficiently, low frequency AC current is not suitable for end use in consuming facilities. Thus, prior to end use power delivered by a utility has to be converted to a useable form. To this end a typical power xe2x80x9cconditioningxe2x80x9d configuration includes an AC-to-DC rectifier that converts the utility AC power to DC across positive and negative DC buses (i.e., across a DC link) and an inverter linked to the DC link that converts the DC power back to three phase AC power having an end useable form (e.g., three phase relatively high frequency AC voltage). A controller controls the inverter in a manner calculated to provide voltage waveforms required by the consuming facility.
Motors and linked loads are one type of common inductive load employed at many consuming facilities and, while the present invention is applicable to several different load types, in order to simplify this explanation an exemplary motor and load will be assumed. To drive a motor an inverter includes a plurality of switches that can be controlled to link and delink the positive and negative DC buses to motor supply lines. The linking-delinking sequence causes voltage pulses on the motor supply lines that together define alternating voltage waveforms. When controlled correctly, the waveforms cooperate to generate a rotating magnetic field inside a motor stator core. The magnetic field induces (hence the nomenclature xe2x80x9cinduction motorxe2x80x9d) a field in motor rotor windings. The rotor field is attracted to the rotating stator field and hence the rotor rotates within the stator core.
When selecting switches to configure an inverter several inverter requirements have to be considered. For example, among others, switching speed and power handling capabilities are extremely important switch selection considerations. With respect to power handling, given an expected maximum expected bus voltage inverter switches capable of handling the maximum DC voltage must be selected. Thereafter, during inverter operation, the DC bus voltage must be limited to an upper value below the maximum DC bus voltage to avoid destroying the switches.
One way to stop a motor and linked load is to cut off power to the inverter such that the stator field is eliminated. Without power the stator and rotor fields diminish and eventually the rotor slows and stops. While this stopping solution is suitable for some applications, this solution is unacceptable in other applications where motors have to be stopped relatively quickly.
To stop motors more quickly the controls industry has adopted several solutions. One widely used stopping technique is to control the inverter to provide a negative torque on the motor and cause an expedited linear deceleration. To this end, the inverter switches can be opened and closed in a controlled sequence calculated to have the stator field lag the rotor field. Because the rotor field is attracted to the stator field, the lagging stator field applies a reversing or negative torque on the rotor. When such a reversing torque is caused, the rotor and stator operate like a generator and, instead of drawing power from the DC bus, provide power back through the inverter switches to the DC bus. During transit back through the inverter switches some of the power is dissipated by the inverter as heat.
Some rectifier configurations are controlled such that power provided back to the DC bus can be provided back to the utility lines through the rectifier. These configurations are commonly referred to as xe2x80x9cregenerativexe2x80x9d drives as power is xe2x80x9cregeneratedxe2x80x9d back to the supply.
The industry has developed numerous methods for maintaining the bus voltage level below the maximum level including (1) disabling the inverter (e.g., over voltage fault disablement), (2) extending the deceleration ramp and (3) using a power dissipating device (e.g., a dynamic brake). While each of these methods can be used to limit the DC bus voltage, each method has one or more shortcomings.
With respect to disabling the inverter, unfortunately, when the inverter is disabled motor control and the deceleration torque on the motor are disrupted. With respect to extending the deceleration ramp, extending the ramp results in an extended stopping period.
An exemplary dynamic brake includes a braking resistor, a switch, a switch controller and a DC bus voltage sensor. The switch and resistor are in series across the positive and negative DC buses and the switch is linked to the controller. The controller monitors the DC bus voltage via the sensor and, when the bus voltage exceeds a specified voltage limit, closes the switch. When the switch is closed, current passes through the resistor and the resistor dissipates power from the inverter.
Typical dynamic brake controllers control the brake switches in a hysteric fashion such that, if the specified voltage limit is 750V, after the switch is closed at 750V, that switch will remain closed until the DC bus voltage reaches some lower value (e.g., 735V). The range between the DC bus limit (e.g., 750V) and the lower hysteric value (e.g., 735V) is referred to hereinafter as a xe2x80x9cbrake hysteric rangexe2x80x9d or brake range DCR.
The peak power that the brake must absorb to limit the rise in bus voltage and avoid an over voltage trip during deceleration is one operating characteristic that must be considered when configuring a dynamic brake for a drive. For a rotating motor and load the stored energy is proportional to the square of the speed of rotation. Thus, during deceleration, the amount of power returned to the DC bus is highest (i.e., is at its peak) at the beginning of the deceleration period. During a typical linear deceleration the average power sent back to the DC bus is about one half of the peak power. Thus, the peak power exists for a very small amount of time and dynamic brakes are routinely underutilized during deceleration. This is because brakes must be designed to handle the peak power.
Even where system configurations include dynamic brakes, if power delivered back to the DC bus is excessive such that the braking resistor cannot dissipate sufficient power even when full on (i.e., when the switch is constantly closed), an over voltage fault may occur and the inverter protection feature will turn off the inverter to protect the inverter components. To this end, deceleration power is also proportional to the motor/load inertia such that a larger inertia will deliver more power to the DC bus than a relatively smaller inertia during deceleration.
To reduce the possibility of an over voltage condition many inverter drives include a regeneration power limit (RPL) or deceleration ramp rate that limits the power delivered to the DC bus during deceleration. Unfortunately, because DC link power is a function of motor/load inertia the optimum RPL or deceleration ramp varies as a function of system inertia and therefore the xe2x80x9coptimum RPL or deceleration rampxe2x80x9d is only optimum under very specific conditions. In many applications the RPL or deceleration ramp is set via a trial and error commissioning procedure to accommodate the highest inertia associated with the motor/load which typically occurs when the motor/load is rotation at a maximum velocity. Where inertia changes (e.g., the load is changed or motor/load speed is altered), to maintain optimum operation, the RPL or deceleration ramp must be manually adjusted.
Therefore, there is a need for an inverter braking system that can stop a load (e.g., motor and connected load) within a given time period that requires a relatively small and inexpensive brake mechanism. It would also be advantageous to have such a system that does not require manual RPL or deceleration ramp adjustments to facilitate optimum operation.
The present inventors have recognized that if the goal is to slow or stop a motor and mechanically couple load in a given time period without regard to deceleration linearity, a technique can be employed such that the power returned to the DC bus and therefore the dynamic brake is initially limited and can be kept essentially constant throughout a braking cycle. In this case, overall brake size in terms of current and power handling can be significantly reduced since sizing the brake for peak power is no longer required.
According to the present invention, with a motor/load running at an initial relatively high speed, after a deceleration command is received, a decelerating torque is applied to the motor increasing the DC bus voltage. When the bus voltage increases above the DCR level, the brake is 100% utilized (turned on 100% of the time). At this time the brake resistor is connected across the DC bus and is dissipating power at the brake maximum level. At this point the controller limits the decelerating torque to stop the increase of DC bus voltage and avoid a DC bus over voltage fault. As the motor/load speed decreases the decelerating torque can be proportionally increased to maintain a constant 100% utilized brake power. The decelerating torque can be increased until a drive torque, current or decelerating ramp limit is reached. When the limit is reached the brake power decreases dropping below the 100% utilization level. Since the dynamic brake is operating at constant 100% utilization throughout essentially the entire deceleration a significantly smaller (therefore lower cost) brake can be used for a given deceleration time.
Thus, one object of the invention is to provide an inverter braking mechanism wherein fast deceleration of an inverter-controlled load is accomplished with a reduced size brake mechanism. To this end, when a deceleration command is received, the inverter is operated to control the power to the brake and maintain the brake essentially full on over a majority portion of the deceleration period.
Another object of the invention is to eliminate the need for manual RPL or deceleration ramp adjustments. This object is accomplished by setting the DC bus voltage limit DCL such that the maximum DC bus voltage is limited irrespective of load characteristics. Thus, loads can be swapped and system inertia altered and the inventive control scheme automatically adjusts to provide optimum control (i.e., 100% brake utilization) and protection against a bus over voltage condition.